How porpoise sounds helped researchers test acoustic devices

A team of scientists used playbacks of recorded and artificial porpoise clicks to develop an adaptable method to assess the area in which acoustic monitoring devices can reliably detect these sounds

Researchers need to know how far away they can expect acoustic data loggers to capture the sounds of target animals to estimate the density of those animals from the recordings.

The cetacean data loggers could reliably detect the click signals up to nearly 200 meters (656 feet), which translated to a circular sampling area of 11 hectares (27 acres) per device.

The data logger algorithms could correctly classify the clicks as porpoise sounds only up to 72 meters (236 feet), representing a reliable sampling areas of just 1.6 hectares (4 acres) that could be used to estimate the density of a specific species, an issue affecting researchers working with more than one echolocating species.

An international team of scientists has developed a method to assess the detection area of acoustic monitoring devices.

These instruments, which can record the calls and other sounds of animals 24/7, have enhanced research on various cryptic yet vocal species, including bats, birds, frogs, bees, and tigers. Passive acoustic monitoring (PAM) units also work underwater to detect the presence or abundance of species such as whales and dolphins.

An acoustic device counts only those animals that vocalize within the range of its sound detectors, missing any animals that call beyond its reach. However, few studies have quantified the percentage of vocalizations in a given area that these devices actually capture.

A harbor porpoise surfacing in waters off Denmark. These small cetaceans usually stay close to coastal areas, estuaries and harbors. Image by Erik Christensen via Wikimedia Commons, CC 3.0.

“Although animals can be recorded using acoustic devices,” team lead Hanna Nuuttila, the SEACAMS scientific officer at Swansea University’s College of Science, said in a statement, “it can be very difficult to quantify the exact ranges and detection areas for acoustic data loggers, something which is crucial if acoustic data is to be used to estimate animal abundance.”

The research team played recordings of harbor porpoise (Phocoena phocoena) vocalizations at a range of volumes and distances from a set of PAM devices known as cetacean click loggers (C-PODs), to estimate how these two factors affect the ability of these units to record the sounds. They published their findings last week in the journal Methods in Ecology and Evolution.

“This study describes a playback experiment and an analytical method to estimate the effective detection range and area for passive acoustic cetacean click-loggers,” Nuuttila said.

A underwater testing protocol

To use PAM devices successfully, researchers need to know the maximum distance at which these devices can detect the sounds of the target animal. Like the concept behind distance sampling, the farther away an animal is from the device, the lower the probability its vocalization will be detected.

Estimating the actual number of animals in a given area based on a number of animals “heard” by a project’s acoustic monitoring units therefore requires a model that takes into account the animals within each unit’s detection area and those just beyond their reach. To estimate the density of those animals from the recordings, researchers need to calculate the detection area of their acoustic devices.

Cetaceans (whales and dolphins) navigate and feed using a biological sonar called echolocation. The animals emit high-pitched clicking sounds and listen to the echoes of those sounds bouncing off various nearby objects, such as prey or each other. The time between the clicks and the returning echo tells them the distance and location of nearby objects.

The C-PODs are underwater acoustic data loggers that pick up and record the echolocation clicks, allowing cetacean researchers to monitor their study animals. The research team determined the range of 15 frequently used C-POD devices moored roughly 1.5 meters (5 feet) above the sea bed off the coast of Wales.

They played back artificial and real recorded harbor porpoise sounds from an inflatable boat drifting, with the engine turned off, across the experiment area. Both types of recordings included clicks of varying amplitude and frequency that were within the range of real harbor porpoise vocalization.

By playing back the sequences of clicks at set distances and sound source levels, the researchers could estimate the proportion of sounds captured by each device and compare that to the distance from the source.

They tested playbacks of the real recordings from 590 different distances, ranging from 0 to 640 meters (2,100 feet, twice as tall as the Eiffel Tower in Paris), with a directional transducer that rotated side to side to imitate the sweeping motion porpoises often make with their heads. They analyzed the recordings of the click sequences made by the C-PODs to see first if the devices detected the sounds and also if they identified the recordings as originating from a porpoise.

Assessing probability

The researchers analyzed 343 recordings of artificial playbacks and 409 of real porpoise sounds and found that more powerful playbacks played more closely to the data logger were more likely to be detected and identified as porpoise sounds. For example, the probability of a C-POD logger detecting the recording fell sharply for playbacks made between 100 and 300 meters (330 and 980 feet) from the C-POD.

Fitted probability curves for the detection of artificial playback clicks at different distances from the sound source, for volumes between 176 and 149 decibels (dB) for C-PODs at two study locations (1A and 1B). Each line depicts the fitted probability for one dB value. The probability of the C-POD detecting sound at each volume dropped substantially between 100 and 300 meters from the source. Image is Figure 4 of Nuuttila et al (2018), “Estimating effective detection area of static passive acoustic data loggers from playback experiments with cetacean vocalisations.”

Using statistical models, the researchers estimated the effective detection radius (EDR), the distance that the data loggers could reliably detect individual clicks and click sequences of the two types of signals.

The maximum distance a data logger captured the sounds was 566 meters (1,857 feet), though the average EDR among the data loggers was just under 200 meters (656 feet) for both real and artificial porpoise click signals. The researchers calculated that both artificial and recorded porpoise clicks could reliably be detected within a circular area of 11 hectares (27 acres), a.k.a. the effective detection area (EDA).

For the algorithms in the data loggers to recognize the caller’s species, however, the sound source had to be closer to the devices, an issue that would affect researchers working in areas with more than one echolocating species. In this case, the algorithms’ EDR for clicks correctly classified as porpoise sounds was just 72 meters (236 feet). In other words, each C-POD could be expected to provide a circular sampling area of about 1.6 hectares (4 acres) to accurately estimate the density of a specific species.

A harbor porpoise underwater. These porpoises have small pointed flippers and lack the dolphins’ beak. The dorsal fin is small and triangle-shaped, and they tend to be a bit chubbier-looking. Both groups echolocate. Image by Ecomare/Salko de Wolf Den Hoorn Texel via Wikimedia Commons, CC 4.0.

The scientists say in their paper that their analytical methods can be used to estimate the EDA of any logger used in abundance estimates.

They stress the importance of accounting for both biological and environmental factors affecting vocalizations when determining the detection area of an acoustic monitoring project. In this study, for example, they found that the variation in detectability by the C-POD data loggers was influenced more by the environmental conditions at the deployment sites than the difference among the devices.

They say that these differences “are likely due to a combination of factors including C-POD sensitivity, subtle differences between deployment sites such as unexpected boulders or troughs in the seabed or variation in the substrate type, the deployment depth, and most importantly the added variability in the transmitted signal, due to hydrophone directionality and the added movement by the operator mimicking the side-to-side movement of the porpoise head.”

Since ambient noise affects cetacean calling strength and frequency, the researchers recommend measuring the source levels of sounds in the area of interest before using acoustic data loggers to estimate a species’ abundance. As shipping and other human noises increasingly infiltrate marine environments, marine mammals may alter their vocalizations, further affecting both their study and their behavior.